I'm definitely of the mindest that the older bro doesn't deserve forgiveness, simply because he's a fucking loser. That said, it seems like the older SISTER is the true mastermind and the real POS in this. That said, with this already being generic as fuck storytelling, at least plotwise, I also wouldn't be surprised the older sister did it for a number of reasons the author pulls out of their ass. 1) She was trying to "protect" her cliche. Which also for whatever reason means raping her little sister mentally. Or 2) The lil sis was actually OP and they had to seal it away to "protect" themselves and the only other option was to kill her. 3) She wants Lily's power for herself.
And, lmao, I swear if the author pulls the "but wait! It wasn't the older sister at all! She was manipulated and the real villain is I, Dio!" cliche no manner of fluffy rom-com could save this, IMO anyway.
Welp, that was another pointless rant by yours truly, hope you hated it as much as I did writing it.